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Changewheels and Screwcutting
Myford Series 7 10 changewheels of all years are 20 D.P. with a 14.5-degree pressure angle, 3/8" thick (0.375") with a 5/8" bore (0.625") and 1/8" keyway (0.125"). Supplied as standard with an ML7 or Super 7, the changewheel set comprised: 2 x 20, 25, 30, 35, 38, 40, 45, 50, 55, 60, 65, 70 and 75t. To cut a wide range of metric threads required, in addition, just two 21t wheels.
Edit: The above shows the arrangement without a quick change gearbox. You might be able to use this information together with the quick change gearing chart to figure out what you need to do for specific metric thread pitches. It is only compounded ratios either way. If you have a 8 TPI lead screw and are cutting a 8 TPI thread, then the gear train is making an eight to one reduction ratio from spindle to lead screw...
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http://www.lathes.co.uk/myford/ (lots more about the super 7 at the link)
http://www.myford.co.uk/acatalog/screw cutting big.jpg (screw cutting chart for super 7)
Edit: Here is the quick change page:
http://www.lathes.co.uk/myford/page16.html